The present invention relates to automatic gain control circuits. The embodiment described illustratively herein is particularly suitable for audio signals, but it will be understood that the present invention will find applicability in other environments.
Typical AGC circuits pass inputted signals through a variable gain amplifier to an output. A feedback circuit adjusts the gain on the amplifier, and generally the gain follows a voltage on a capacitor. Usually a comparison circuit receives signals representing the level of the output signal and compares the representative signal against a threshold value. If the signal level is too high, the comparison circuit permits the capacitor to discharge to ground, and the gain will decrease. If the gain is too low, the comparison circuit will present a high impedance to the capacitor. The voltage on the capacitor will increase through the action of current flow from a voltage source tied to the capacitor through a resistor. The voltage on the capacitor will develop according to the well-known RC curve which is a negative exponential curve, i.e, its second derivative with respect to time is negative.
As a result, when the gain is increasing in prior art AGC circuits, the RC curve and hence the gain change is steep initially and progressively decreases in slope. This characteristic tends to distort low frequency components of the signal. The sound which results is unpleasant to the ear. To compensate for this, most AGC circuits increase the RC time constant so that the initial rapid change in gain is tolerable, which lengthens considerably the recovery time of the circuit.
However, this solution has not been entirely successful. When a signal at a high level drops to a low level, the low level audio signal will be entirely lost for a time, due to the slow recovery time.
It is therefore an object of the present invention to provide an AGC circuit which has a fast recovery time without substantially distorting low frequency components of the signal.